When i revised i did 30 minutes at a time with a 5 minute break as the brain cannot be force fed knowledge.
Also a tip for you. The brain needs to repeat it 5 times, its boring but it works. i dont mean say it 5 times in your head i mean; read it. Say it. Test it. Keep doing the bits your struggling.
If you can, do it with friends but if you know your going to be distracted work on your own in silence. Best place is after school as there is no distractions.